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Summers in the City Worse Than Ever

Grandpa may recall hot summers from his youth, but chalk those memories up to youthful hormones. The dog days of summer have been more brutal in recent years for U.S. cities and towns than they were decades ago, researchers have found. A half-century worth of data, analyzed in tomorrow's issue of Nature, reveals that the incidence of heat waves has been steadily mounting across the nation, posing a threat to health.

"We've known for a while that temperatures are increasing," says National Oceanic and Atmospheric Administration research meteorologist Dian Gaffen. "But we haven't known much about how the extreme events have been changing." To see how recent heat waves stack up against those of years gone by, Gaffen and colleague Rebecca Rosa gathered temperature and humidity data from 113 locations across the country, mostly airports, going back to 1949.

The researchers pored over days-long extremes in heat stress--a double whammy of high temperature and humidity that can trigger heat strokes in the elderly and other vulnerable people. From Miami to Philadelphia, the duo found that, on average, the frequency of heat waves had nearly doubled. Moreover, the annual number of hot days--defined as average temperatures exceeding between 25 and 30 degrees Celsius, depending on location--rose by almost two a decade. The results are "consistent with warming of the global atmosphere," says Gaffen, although she emphasizes that precise reasons for a hotter United States are unknown. But the trends held up from airport to airport, suggesting that local perturbations cannot account for the hot streaks.

Experts laud the researchers' analysis of both temperature and humidity data. This is a much-needed approach, because climate change "really deals with all aspects of weather," says Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research in Boulder, Colorado.